The production of chlorine by electrolysis of alkali chloride solutions, with particular reference to sodium chloride and potassium chloride (hereinafter “brine”) is currently carried out according to three different processes, namely, the ion-exchange membrane process, the porous diaphragm process, and the mercury cathode process. The latter type, based on a long-known technology, has experienced a continuous improvement in the cell structure (Ullmann's Encyclopaedia of Industrial Chemistry, VCH, Vol. A6, pag. 416) essentially directed to decreasing the electric energy consumption and lessening the release of mercury into the environment.
The problem of reducing the energy consumption was tackled with success by replacing the original graphite anodes with titanium anodes activated with a particularly effective coating based on oxides of platinum group metals. The activated titanium anodes are also characterised by a long operative lifetime allowing a substantial reduction in the amount of cell shut-downs which were quite frequent in the case of the corrodible graphite anode. Since the maintenance shut-down is a crucial operation as regards the mercury release into the environment, the benefit obtained under this standpoint is apparent.
A further mercury leak reduction was also obtained by the routinary use of recrystallised salt which permits minimising the quantity of mercury-polluted muds purged from the brine purification section, although introducing an additional cost. As a consequence of these measures it can be nowadays demonstrated that the mercury release form a well-designed and correctly handled plant amounts to no more than 3 grammes per tonne of product chlorine versus a value of 10 grammes of about ten years ago (Ullmann's Encyclopaedia of Industrial Chemistry, VCH, Vol. A6, page 424). Such an amount could be further reduced if the frequency of maintenance shut-downs was further decreased: for the time being, such frequency is substantially imposed both the need of periodically cleaning or replacing the inlet and outlet cell end-boxes.
The term inlet end-box indicates the section connected to the initial part of the cell body: such section is directed to ensure the uniform non-turbulent brine and mercury admission into the cell body, as necessary to prevent harmful short-circuits. Examples of inlet end-box design can be found in the prior art.
In accordance with prior art techniques, end-boxes are made of carbon steel lined with various types of synthetic or natural rubbers, usually vulcanised by means of a suitable final thermal treatment in an autoclave.
With the design currently used for the inlet and outlet end-boxes, the accumulation of foreign material, —for instance powders of oxides or other insoluble products, salt scales and the so-called mercury butters, takes place in correspondence of the dead zones, with negative effects on the regularity of the mercury and brine flows and the relatively quick deterioration of the lining as a consequence of the combination of the aggressiveness of fluids, in particular chlorine, with the temperature which may easily reach peaks above 100° C. A further source of deterioration is given by the progressive embrittlement which turns the coating rather sensible to the start-up and shut-down thermal transients. All this forces the operators to carry out periodic shut-downs to proceed with the end-box replacement or with difficult operations of manual cleaning, during which the mercury vapour release in the working environment is practically unavoidable. The replacement in its turn introduces an additional problem which makes this expensive operation even more onerous since the vulcanised rubber lining of the disassembled end-box contains non negligible amounts of mercury and highly toxic products such as dioxins and furanic compounds generated by reaction with chlorine, entailing a remarkable cost for their disposal.
To overcome these difficulties, several types of lining provided with higher chemical inertia and applied with different procedures were proposed. In one example, the use of fluorinated polymers such as polyvinylidenfluoride (PVDF), polychlorotrifluoroethylene (CTFE) and tetrafluoroethylene-hexafluoropropylene (FEP) copolymer is disclosed.
The utilisation of the application procedures of the prior art can, for instance be practiced for lining the sidewalls of the cell body, while this is practically impossible for the end-boxes due to their very complicated structure involving the presence of several edges.
An innovation which found a good user acceptance consists of inlet and outlet end-boxes integrally made of a plastic material optionally reinforced with glass, Kevlar® or carbon fibres. One interesting polymer in this regard is polycyclopentadiene, commercialised for example by BF Goodrich under the trade-mark Telene® and characterised by high chemical resistance to chlorine even at high temperature and by the advantage of not generating noxious chlorinated products as occurs with the various rubber types of common industrial application. The drawback of this solution (characterised by operative lifetimes around 6-7 years, while the duration of the rubber linings does not exceed 3 or 4 years) is associated to the operative temperatures which, as mentioned above, may also exceed 100° C. In these conditions the mechanical characteristics of the polymers, even of reinforced type, are rather poor and on the other hand the mechanical solicitations due both to the weight of mercury and brine contained in the end-boxes during operation and to the thermal expansion are high: it follows that, in order to prevent harmful deformations and in the worst of cases hazardous fissures, the end-boxes made of polymer material must be suitably overdimensioned in terms of thickness and also designed so as to incorporate adequate reinforcing elements with an associated complexity of the moulds. The obvious consequence is represented by a remarkable cost, which up to now has limited their successful commercialisation.
The invention is directed to overcome the above limitations of the mercury cathode cell inlet and outlet end-boxes of the prior art.